The present invention relates to an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus and, more particularly, to a developing device included in such an apparatus for developing a latent image electrostatically formed on a photoconductive element, or image carrier, by toner.
Today, it is a common practice with an electrophotographic image forming apparatus to use an image carrier implemented as a photoconductive belt. The belt is held in contact with a developing roller which is included in a developing device and plays the role of a developer carrier. This kind of arrangement is advantageous in that the wear of the belt is relatively slow due to the elasticity of the belt. However, the belt is problematic from the cost and conveyability aspect. This problem cannot be eliminated without further increasing the cost. For a miniature and inexpensive apparatus, a photoconductive element in the form of a drum is advantageous over the photoconductive belt. However, a photoconductive drum is not durable since a photoconductive layer formed thereon noticeably wears due to friction between it and a developing roller.
A single component type toner, i.e., toner promotes the decrease in the size of the developing device and basically eliminates the need for maintenance, as well known in the art. However, the problem with this kind of developer or toner is that the reliability achievable therewith is poor, and that it cannot be uniformly charged with ease and eventually contains grains charged to the opposite polarity. Toner charged to the opposite polarity contaminates the background of a toner image. The prerequisite with the developing roller, or developer carrier, is that uniformly charged toner forms a layer thereon. Should the amount of toner on the developing roller be excessive, non-charged toner occurs and turns out oppositely charged toner. Further, when the ratio of the linear velocity of the developing roller to that of the photoconductive element is increased, it is likely that a scavenging force acting on the toner becomes excessive for the velocity difference or that the photoconductive element and toner rub against each other, resulting in undesirable charging. In the case where toner is charged by friction between a toner regulating member and the developing roller or by charge injection, the amount of toner which can be uniformly charged, i.e., the amount of toner to deposit on the developing roller for a unit area is limited. Amounts of toner exceeding this limit would increase the ratio of oppositely charged toner. Therefore, it is not always practicable to deposit the same amount of toner on the developing roller as required on the photoconductive element. At the present stage of development, a required amount of toner is not attainable unless the amount of toner on the developing roller and the linear velocity ratio of the developing roller to the photoconductive element are suitably increased., i.e., the amount of toner on the developing roller and the linear velocity ratio are well balanced. Even this kind of approach, however, cannot produce desirable images at all times.